1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and the like, which is of electrophotographic type or electrostatic recording type wherein an electrostatic latent image formed on an image bearing member is visualized by developing it with toner, and more particularly, it relates to an image forming apparatus having a toner density control means for properly controlling toner density or toner concentration of two-component developer comprised of toner, and carrier particles for applying frictional charge to the toner.
2. Related Background Art
As is well-known, the toner density of the two-component developer (i.e., rate of toner weight with respect to the total weight of the mixture of the carrier particles and the toner) is very important for stabilizing image density and image quality. The toner in the developer is consumed during the developing operation, thereby changing the toner density. Thus, the use of a toner density control device is required to properly detect the toner density at any time, to replenish new toner in the developer in accordance with the change in the toner density and to always keep the toner density constant.
In conventional toner density control devices, an optical detection system for detecting the toner density by detecting an amount of light reflected from the developer, an inductance detection system for detecting the toner density by detecting inductance of the developer, or a capacity detection system for detecting the toner density by detecting electrostatic capacity of the developer has been used. However, in such conventional toner density control devices, since indirect information such as the reflected light amount or inductance of the developer was used, it was impossible to detect the correct toner density of the developer.
For example, when the amount of light reflected from the developer is detected, although the reflected light amount is varied in accordance with the change in the toner density of the developer, it is also varied in accordance with the change in density of the developer. Particularly, when the developer including toner having the larger building-up change in its frictional charging charge is used, although the density of the developer in a developer container is initially high, during the continuous formation of images, the density of the developer will become rough gradually as the frictional charge of the toner is being built-up, with the result that the reflected light amount will be decreased even if the toner density is constant (not varied). Consequently, the toner density control device will judge as if the toner density is decreased, and, thus, replenish new toner to the developer, with the result that the actual toner density will be increased.
In the above, while the optical detection system for detecting the amount of light reflected from the developer was described, also in the conventional detection systems for indirectly detecting the toner density of the developer (for example, inductance detection system), since they are apt to be influenced upon the change in the density of the developer (other than the change in the toner density), the correct toner density of the developer cannot be detected, thus making the control of the toner density with high accuracy difficult or impossible.